One of the known methods for preparing polyorganosiloxane foams involves the reaction of polyorganosiloxanes containing silicon-bonded hydroxyl groups with organosilicon materials containing silicon-bonded hydrogen atoms in the presence of a suitable catalyst. Reactants containing silicon-bonded vinyl radicals can be included in the composition to enhance the properties of the final cured foam. Foams of this type are disclosed in U.S. Pat. No. 3,024,210 to Weyer, which issued on Mar. 6, 1962, U.S. Pat. No. 3,070,555 to Bruner, which issued on Dec. 25, 1963, U.S. Pat. No. 3,338,847 to Nitzche et al., which issued on Aug. 29, 1967 and U.S. Pat. No. 3,923,705 to Smith, which issued on Dec. 2, 1975.
In the absence of an inhibitor, compositions containing hydroxyl groups and silicon-bonded hydrogen atoms begin to react at ambient temperature almost as soon as the reactants containing these groups are combined. Inhibitors such as acetylenic alcohols will at least partially suppress the reactivity of the composition at 25.degree. C., and the composition must be heated to temperatures of 75.degree. C. or higher to produce a foam. Such compositions cannot be used as storage stable one-part formulations for preparing cured foams at room temperature.
The preparation of foams by the action of a blowing agent within moisture curable room temperature vulcanizable (RTV) polyorganosiloxane compositions is known. These compositions typically contain a liquid hydroxyl endblocked polydiorganosiloxane and a liquid or solubilized crosslinker containing 3 or more silicon-bonded groups which undergo hydrolysis readily at room temperature and in the presence of atmospheric moisture.
The relatively slow curing rate that characterizes RTV compositions is particularly disadvantageous for the preparation of foams. At the relatively low viscosity desired to facilitate blending and transfer of the foamable composition, the partially cured foams tend to collapse when the introduction or evolution of blowing agent ceases, and at least a portion of the uncured liquid reactants in the cell walls drains from the foam. In addition to causing a collapse of the foam, the liquid which drains cures to a solid rubber.
In U.S. Pat. No. 4,368,279, which issued on Jan. 11, 1983, F. T. Modic and B. E. Boudreau teach that the collapse of foams obtained by beating air or other gaseous blowing agent into RTV polyorganosiloxane compositions can be avoided by placing the uncured foam under a vacuum equivalent to a maximum pressure of 79.8 kPa for at least 5 minutes. The curable compositions disclosed by Modic and Boudreau contain a silanol terminated polydiorganosiloxane base polymer, a crosslinking agent, a curing promoter and, in the case of one-part compositions, a small amount of water.
While the use of vacuum during curing may be practical for machine made foams such as slab stock and molded foams, this technique cannot be used when the foam is formed within a cavity of a building or at other locations where it is not feasible to maintain the foam under vacuum during curing. In addition, the necessity of mixing in a gaseous blowing agent at the time the foam is formed requires that mixing and aerating equipment be transported to the location where the foam is to be installed. For some applications, particularly those requiring relatively small amounts of foam at relatively remote locations, such equipment would not be practical. In these instances it would be considerably more convenient to employ a one-component foamable composition, including a blowing agent, packaged in a container that can be easily transported to the application site and which is capable of repeatedly dispensing the foamable composition without the need for additional processing steps or ingredients.
U.S. Pat. No. 4,229,548, which issued on Oct. 21, 1980 to Sattleger et al. discloses a 2-compartment aerosol type container for storing and dispensing a foamable polyorganosiloxane composition. The container consists of a flexible walled inner compartment containing a foamable RTV composition that includes a hydroxy-endblocked polydiorganosiloxane, a curing agent and, optionally, a gaseous blowing agent and an outer compartment containing a moisture free inert gas under a pressure of from 0.2 to 3.0 megapascals. The container is equipped with a valve through which the foamable composition is dispensed under the presence of the gas confined in the outer compartment of the container. The formation of polyorganosiloxane foams by dispensing a one-part moisture curable RTV polyorganosiloxane composition stored under pressure in a 2-compartment container is also taught in German published application Nos. 2,909,443 (published Sept. 18, 1980) and 2,911,971 (published Oct. 9, 1980).
Foams produced by dispensing silica-filled RTV polyorganosiloxane compositions, including a blowing agent and/or a propellant, from pressurized containers, such as aerosol cans, are characterized by densities from 0.58 to 0.81 g./cc. The need to minimize collapse of the cellular structure in partially cured foams and the resultant increase in density by the use of vacuum, by heating to accelerate curing or by other means requiring additional processing steps may more than offset the advantages achieved by using foamable compositions packaged in portable pressurized containers such as aerosol cans.
The art discloses additives for reducing the density of polyorganosiloxane foams formed by the reaction of silicon-bonded hydroxyl groups with silicon-bonded hydrogen atoms. U.S. Pat. No. 4,026,845, which issued on May 31, 1977 to Y. K. Kim et al., teaches using known fluorine-containing surfactants for this purpose. The surfactants contain fluorine atoms bonded to carbon, and include both organic and organosilicon compounds.